1. Field of the Invention
The subject of the invention is an element made of composite material, comprising at least one assembly intended to ensure electrical conductivity through the element, the composite material being of the type comprising an organic matrix which requires a heat treatment in order to reach a critical rigidifying and/or shaping temperature and is reinforced with mineral or organic fibres, the critical temperature being a polymerization temperature in the case of a thermosetting matrix and a shaping temperature, for example shaping by moulding, in the case of a thermoplastic matrix.
The subject of the invention is also the process for manufacturing this element, as well as its use in aeronautics.
2. Description of the Prior Art
The "metallization" of the elements or pieces made of composite materials, that is to say the measures taken to ensure electrical continuity through these elements or pieces, is a necessary requirement in the aeronautical field in order to ensure, in particular, protection of the onboard electrical equipments from electromagnetic interference when the aircrafts carrying these equipments are, at least partly, produced using such elements. The object of the invention is, among others, to allow the electrostatic charges, which may originate from electrical stresses for example, to flow away.
In practice, an electrically conductive mesh is interposed in a stack of layers of composite materials, these layers being intended to form a fuselage panel for example, something which amounts to conferring the properties of a Faraday cage on the fuselage of an aircraft produced in this way. Thus, the interior of the fuselage becomes electrically isolated, the mesh allowing the electric charges to drain away.
However, such a structure turns out not to be sufficient to allow high-intensity currents to pass, for example those due to lightning. In this case, the electrical discharge may not be completely drained away by the simple mesh and it runs the risk of adversely affecting the composite element in question and of putting the aircraft in danger.
As a general rule, the remedy consists in arranging in addition, in the stack of layers of composite materials, copper shims, generally in the form of narrow tapes.
Experience shows that these shims, inserted into the composite materials, deform during the heat treatment which the organic matrices of these materials undergo (during the hot polymerization in the case of thermosetting matrices or during hot shaping and/or moulding in the case of thermoplastic matrices). During this operation thermal stresses appear. This may result, in particular, in disbondments or fractures which adversely affect the characteristics of the elements in question.
This may also result in undulations in the copper strips. These undulations have the effect of preventing electrical contact between two successive copper strips. Consequently, such an arrangement may not allow reliable metallization of the elements made of composite materials.
The current solution necessitates limiting these copper tapes to a length of from 0.8 to 1 m approximately, and ensuring electrical continuity by a local overlap by a few centimetres of the copper strips, taken in pairs. As a general rule, this procedure prevents the undulations in the copper tapes during heat treatments.
However, during polymerization or shaping heat treatment, the resins may insinuate between the copper bands in the overlap zones and thus impair the contacts. Consequently, it is no longer possible to ensure proper electrical conduction. It therefore becomes necessary frequently to carry out work on this kind of assembly in order to effect repairs by soldering. Provision may also be made to instal screws or rivets in order to keep the copper strips in contact in the overlap zones, but the results are never excellent (oxidation, increased electrical resistance, etc.). In addition, these operations require time and labour and are consequently expensive.